We’re back to our pistol caliber carbine “build” project. In Part 1, we suggested a lower for our AR-plan gun, as well as a possible handguard (Brigand Arms Blade) and compensator (SJC Titan) to terminate the barrel.
Part 2 had our recommendation of the Spartan Side Charge upper. We’ve had the chance to see or use these in other circumstances with good results, and particularly like the way they “salvage” some older 9 mm-specific lowers that don’t have last-shot hold-open capacity. By cleverly moving this functionality to the left side of the upper receiver, folks in our shoes can stop coveting new lowers—at least temporarily. For many folks, it’s a “meh” sort of deal, but if you fancy a little USPSA or similar PCC competition, it can be an occasional disadvantage.
Our first choice was the JP that will not “wallow” holes in either the lug or lower; AccuWedge is an absurdly easy—though not permanent—fallback. Photo by A1F Staff
Naturally, a barrel must shortly follow upper selection, or “bang” will be unpleasantly delayed. Our friends at Faxon and JoeBobOutfitters have both supplied enticing examples for our experiment. To say we went to our work with zest would be an understatement.
And we immediately encountered an annoying problem: In a pair of configurations that had been all but flawless in previous uses, we started getting an alarming number of “bump” fires. Lots of folks will have experience with these, as there are stock and trigger mechanisms for many firearms that actually excite this “feature,” namely, the use of recoil motion to initiate another shot.
We aren’t fans, as a rule. Our particular occurrences weren’t easily traceable as to cause, but nevertheless unsafe. Having changed a couple of things at once, we backed out changes to the first—our JoeBob barrel—and started over.
Thus began about three weeks of careful experimentation, and even more careful magazine loading. (Never more than two rounds at a time in a firearm with ignition issues all but guarantees things can be kept under control.) Results were mixed as we tried to control variables as far-flung as bullet/powder/primer combinations, barrel lengths, with and without multiple comps, and buffer/spring rate and bolt weight combinations. We even did trigger swaps. Nuttin’.
In the background, naturally, were also a flurry of emails and phone calls, and all to no avail. Our starting configuration was the only way to get back to correct functioning that met our standards: No bump fires—whatever the actual mechanical source—period. That’s reassuring, certainly, but hardly interesting from the “build” perspective.
In the end, conversations with resident geniuses (or is it genii?) at JP put us back on the right track. One of the company’s triggers—and a 9 mm PCC problem-slayer of major standing—is the JP EZ Trigger System. We’d uninstalled one to test other possibilities without benefit, so back in it went.
It certainly helped, but only when we really locked down on the rifle; by that we mean grab everything as firmly as possible, and drive the rifle into the shoulder. Death grip, in other words. That’s not really comfortable or practical for every single shot, especially given how we want to shoot the carbine eventually (defense and competition).
If you’re an old AR hand, you may see around the last corner. Strange as it may seem, centerfire rifle calibers and especially 5.56/.223 don’t require the upper/lower fit to be all that tight to keep the rifle functioning. Different engagement geometry between the cocking action of the bolt carrier and hammer face give the disconnector plenty of time to latch up.
Blowback PCCs like our build don’t have the same geometry. When you lock the carbine in tight, all is well: Bang, bang, bang, etc. But if the upper/lower fit is a little soft, there’s enough flex to simulate the “bump” version of trigger actuation.
In one of those odd pieces of unexpected symmetry, JP probably had the most elegant solution to the problem, their AR Tension Pin. In a further peculiar twist, however, we were slightly alarmed not to be able to find it anymore on JP’s own website (our link finds it in stock at Brownells, as well as here and here [backorder]).
Tighten that upper/lower fit with ease and trivial cost, but replace the AccuWedge when any “fraying” appears—and it will. Photo by A1F Staff
But to push the build ahead and get to those intriguing barrels and comps, we’ve got a nifty fallback in the AccuWedge. This one we’ve run about 800 rounds over in the last week—including a mishap-free match—and had 100 percent results. Note that you aren’t after a fit that’s so tight it requires tools to get in your gun, so you may want a razor blade or X-Acto handy for judicious trimming. This is not permanent solution in the sense that they do wear out, but who cares: It’s a $4-7 part.
For the long term, JP re-enters the picture with their MicroFit Takedown Pins. Though we (obviously) liked the tension version, these are an advance in several senses, but one in particular: They get you back to toolless entry on your rifle or carbine. Where the tension pin needed a hex key, these install like traditional pins, and go in and out with cartridge nose pressure.
They also bring the ability to adjust both pins if needed. Available in .001” undersized, standard and .001” oversized pairs, they could even be mixed and matched to get an absolutely perfect fit. (We’ve got a set on the way and will report back.)
So remember our travails if you’re following the build. It’s a good plan to have an AccuWedge on hand if there’s even a little wobble between upper and lower, and perhaps to budget a set of JP MicroFit Takedown Pins for a permanent, no-wobble fit should our hiccup appear in your build.
Frank Winn has been studying arms and their relationship to tyranny, meaningful liberty and personal security all his adult life. He has been a firearms safety/shooting instructor for more than 20 years, and earned state, regional and national titles in several competitive disciplines.